refactor: extract shared token package (#46)

## Description

Both the `saccharine` and `lambda` packages need tokenizing and parsing primitives.
This PR extracts shared token infrastructure into a new `pkg/token` package, then wires both languages up to use it.

- Add `pkg/token` with a generic `Token[T]` type, `Scan`, `ScanAtom`, `ScanRune`, `ScanCharacter`, `IsVariable`, `ParseRawToken`, and `ParseList`.
- Refactor `pkg/saccharine` to delegate to `pkg/token`, removing duplicated scanning and parsing helpers.
- Implement `Codec.Decode` for `pkg/lambda` (scanner + parser) using the shared token package.
- Add `iterator.While` for predicate-driven iteration.
- Rename `iterator.Do` to `iterator.Try` to better describe its rollback semantics.

### Decisions

- The `Type` constraint (`comparable` + `Name() string`) keeps the generic token flexible while ensuring every token type can produce readable error messages.
- `iterator.Do` was renamed to `iterator.Try` since it describes a try/rollback operation, not a side-effecting "do".

## Benefits

- Eliminates duplicated token, scanning, and parsing code between languages.
- Enables the `lambda` package to decode (parse) lambda calculus strings, which was previously unimplemented.
- Makes it straightforward to add new languages by reusing `pkg/token` primitives.

## Checklist

- [x] Code follows conventional commit format.
- [x] Branch follows naming convention (`<type>/<description>`). Always use underscores.
- [x] Tests pass (if applicable).
- [ ] Documentation updated (if applicable).

Reviewed-on: #46
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

pkg/token/token.go

@@ -0,0 +1,36 @@
+// Package token provides generic token types and scanning/parsing primitives
+// for building language-specific lexers and parsers.
+package token
+
+// A Type is a constraint for language-specific token type enums.
+// It must be comparable (for equality checks) and must have a Name method
+// that returns a human-readable string for error messages.
+type Type interface {
+	comparable
+	// Name returns a human-readable name for this token type.
+	Name() string
+}
+
+// A Token is a lexical unit in a source language.
+type Token[T Type] struct {
+	Column int    // Where the token begins in the source text.
+	Type   T      // What type the token is.
+	Value  string // The value of the token.
+}
+
+// New creates a Token of the given type at the given column.
+// The token's value is derived from its type's Name method.
+func New[T Type](typ T, column int) *Token[T] {
+	return &Token[T]{Type: typ, Column: column, Value: typ.Name()}
+}
+
+// NewAtom creates a Token of the given type with a custom value at the given
+// column.
+func NewAtom[T Type](typ T, name string, column int) *Token[T] {
+	return &Token[T]{Type: typ, Column: column, Value: name}
+}
+
+// Name returns the type of the Token, as a string.
+func (t Token[T]) Name() string {
+	return t.Type.Name()
+}